Single-Mode Optical Fiber

ABSTRACT

A single-mode optical fiber includes a central core surrounded by an outer cladding. The optical fiber includes at least first and second depressed claddings positioned between the central core and the outer cladding. The central core typically has a radius of between about 3.5 microns and 5.5 microns and a refractive-index difference with the outer cladding of between about −1×10 −3  and 3×10 −3 . The first depressed cladding typically has an outer radius of between about 9 microns and 15 microns and a refractive-index difference with the outer cladding of between about −5.5×10 −3  and −2.5×10 −3 . The second depressed cladding typically has an outer radius of between about 38 microns and 42 microns and a refractive-index difference with the first depressed cladding of between about −0.5×10 −3  and 0.5×10 −3 .

CROSS REFERENCE TO PRIORITY APPLICATION

This application hereby claims the benefit of pending EuropeanApplication No. 11305654.3 (filed May 27, 2011, at the European PatentOffice), which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of optical-fibertransmissions and, more specifically, single-mode optical fibers (SMF).The invention embraces a single-mode optical fiber having reducedattenuation, which can be manufactured from an increased-capacityoptical preform.

BACKGROUND

An optical fiber's refractive-index profile is generally described as arelationship between refractive index and optical-fiber radius.Conventionally, the distance r to the center of the optical fiber isshown on the x-axis, and the difference between the refractive index (atradius r) and the refractive index of the optical fiber's outer cladding(e.g., an outer optical cladding) is shown on the y-axis. The outercladding, functioning as an optical cladding, typically has a refractiveindex that is substantially constant. This outer cladding is typicallymade of pure silica but may also contain one or more dopants.

The refractive-index profile may have a “step” profile, a “trapezoidal”profile, a “parabolic” profile (e.g., an “alpha” profile), or a“triangular” profile, which can be graphically depicted as a step,trapezoidal, parabolic, or triangular shape, respectively. These curvesare generally representative of the theoretical or design profile of theoptical fiber. Constraints associated with optical-fiber fabrication maylead in practice to a profile that is perceptibly different.

An optical fiber conventionally includes an optical core, which has thefunction of transmitting and optionally amplifying an optical signal. Aconventional optical fiber also typically includes an optical cladding,which confines the optical signal in the core. For this purpose, therefractive index of the core n_(c) is typically greater than therefractive index of the cladding n_(g) (i.e., n_(c)>n_(g)). As will beunderstood by those having ordinary skill in the art, the propagation ofan optical signal in a single-mode optical fiber includes a fundamentalmode, typically denoted LP01, which is guided in the core, and secondarymodes, which are guided over a certain distance in the core and theoptical cladding.

Single-mode optical fibers (SMFs) with a step-index profile are oftenused within optical-fiber transmission systems. Such optical fiberstypically possess a chromatic dispersion and a chromatic-dispersionslope that comply with specific telecommunications standards.

Conventionally, so-called “standard” single-mode fibers (SSMFs) are usedfor land-based transmission systems. To facilitate compatibility betweenoptical systems from different manufacturers, the InternationalTelecommunication Union (ITU) has defined a standard reference ITU-TG.652 with which a standard optical transmission fiber (i.e., a standardsingle-mode fiber or SSMF) should comply. The ITU-T G.652recommendations (11/2009) and each of its attributes (i.e., A, B, C, andD) are hereby incorporated by reference.

Among other recommendations for a transmission fiber, the ITU-T G.652standard recommends (i) a mode field diameter (MFD) with a nominal value(e.g., a nominal mode field diameter) of between 8.6 microns (μm) and9.5 microns and a tolerance of ±0.6 micron at a wavelength of 1310nanometers (nm), (ii) a maximum cable cut-off wavelength (λ_(CC)) of1260 nanometers (nm), (iii) a zero-dispersion wavelength (ZDW) ofbetween 1300 nanometers and 1324 nanometers, and (iv) a maximumzero-dispersion slope (ZDS) of 0.092 picoseconds per square nanometerkilometer (ps/(nm²·km)) (i.e., the chromatic-dispersion slope at thezero-chromatic-dispersion wavelength is 0.092 ps/(nm²·km) or less).

The cable cut-off wavelength is conventionally measured as being thewavelength at which the optical signal is no longer single mode afterpropagating over 22 meters in the optical fiber, as defined bysubcommittee 86A of the International Electrotechnical Commission (IEC)in standard IEC 60793-1-44. The IEC 60793-1-44 is hereby incorporated byreference in its entirety.

In most circumstances, the secondary mode that best withstands bendinglosses is the LP11 mode. The cable cut-off wavelength is thus thewavelength from which the LP11 mode is sufficiently attenuated afterpropagating for 22 meters in an optical fiber. The method proposed bythe ITU-T G.652 standard considers that the optical signal is singlemode as long as the attenuation of the LP11 mode is greater than orequal to 19.3 decibels (dB). According to the recommendations of IECsubcommittee 86A in standard IEC 60793-1-44, the cable cut-offwavelength is determined by imparting two loops having a radius of 40millimeters (mm) in the optical fiber, while arranging the remainder ofthe optical fiber (i.e., 21.5 meters of optical fiber) on a mandrelhaving a radius of 140 millimeters.

Optical fibers can have pure-silica cores. The absence of dopant in thecore of a Pure-Silica-Core Fiber (PSCF) makes it possible to limitoptical losses and notably the attenuation at a wavelength of 1550nanometers. A PSCF therefore typically has a cladding formed of silicadoped with fluorine to reduce its refractive index and ensure that theoptical signal is confined within the core.

Conventionally, an optical fiber is drawn from an optical-fiber preformin a fiber-drawing tower. The operation of drawing down an optical fiberto scale includes placing the optical-fiber preform vertically in atower and drawing a strand of optical fiber from one end of the preform.For this purpose, a high temperature is applied locally to one end ofthe optical-fiber preform until the silica is softened, and then thespeed of fiber-drawing and the temperature are continuously regulated tocontrol the diameter of the optical fiber. The optical-fiber preformmust present the same ratio of core diameter to cladding diameter as isto be achieved in the optical fiber drawn therefrom.

An optical fiber may be fabricated from an optical-fiber preform thatincludes a primary preform constituted by a deposition tube of pure ordoped silica in which layers of doped and/or pure silica are depositedin succession in order to form an inner cladding and a central core.Primary preforms of this nature are typically fabricated on a depositionbench. The primary preform is then overcladded (e.g., fitted with asleeve) to increase its diameter and form an optical-fiber preform orfinal preform that is suitable for use in a fiber-drawing tower. In thiscontext, the term “inner” cladding designates the cladding formed insidethe deposition tube (e.g., a substrate tube) and the term “outer”cladding or “overcladding” designates the cladding formed outside thedeposition tube.

Deposition operations inside the deposition tube are typically chemicalvapor depositions (CVD). A CVD deposition is performed by injectingmixtures of gas into a deposition tube and ionizing the mixtures.CVD-type depositions include modified chemical vapor deposition (MCVD),furnace chemical vapor deposition (FCVD), and plasma-enhanced chemicalvapor deposition (PCVD). CVD techniques help to ensure that the OH-peakremains low and that attenuation at 1383 nanometers is thereforelimited.

After layers corresponding to the core and the inner cladding have beendeposited, the deposition tube (i.e., including the deposition layers)is converted into a solid rod by an operation referred to as“collapsing.” This produces the primary preform that is constituted by asolid rod (i.e., a solid rod including the collapsed deposition tube,inner cladding layers, and core layers). The primary preform is thenovercladded, generally with grains of natural silica for reasons ofcost. Overcladding may be performed by plasma deposition in which grainsof doped or pure natural silica are deposited by gravity, melted, andvitrified on the periphery of the primary preform via a plasma torch.

Other techniques also exist for fabricating an optical-fiber preform. Inthis regard, the primary preform may be formed by outside depositiontechniques, such as outside vapor deposition (OVD) or vapor axialdeposition (VAD). During outside deposition techniques, no substratetube is used. Rather, doped and/or undoped silica layers are depositedby directing precursor gases and a torch onto a starting rod.

As noted, various layers of a silica preform may be doped. During doping(i.e., component deposition), dopants are added to silica in order tochange its refractive index. Germanium (Ge) or phosphorus (P) is used toincrease the refractive index of silica. Germanium or phosphorus isoften used for doping the central core of conventional optical fibers.Fluorine (F) or boron (B) is used to decrease the refractive index ofsilica. Fluorine is often used for forming depressed claddings.

Making a primary preform with a large, highly depressed cladding isdifficult. For example, although a high temperature is required formaking silica glass, it is difficult to incorporate fluorine in silicaheated above a certain temperature.

PCVD techniques can be efficiently used to produce a depressed claddinginside a deposition tube. U.S. Pat. No. RE 30,635 and U.S. Pat. No.4,314,833, each of which is hereby incorporated by reference in itsentirety, describe PCVD techniques that allow fluorine to besignificantly incorporated into silica in order to form highly depressedcladdings. These patents describe that a deposition tube, made of puresilica or fluorine-doped silica, is mounted in a glasswork tower. Thetube is then rotated while a gas mixture of silica and dopants isinjected into the tube. The tube crosses a microwave cavity in which thegas mixture is heated locally. The microwave heating generates plasma byionizing the gas mixture injected into the tube. The ionized dopantshighly react with the silica particles, causing the deposition of dopedsilica layers inside the tube. The high reactivity of the dopants,generated by the microwave heating, enables a high concentration ofdopants to be incorporated into the silica layers.

FIG. 1 illustrates a set refractive-index profile of a conventionalPSCF. The depicted profile is a set profile that is representative ofthe optical fiber's theoretical profile. Constraints in the manufactureof the optical-fiber preform and the optical fiber, however, may resultin a slightly different actual profile.

Those having ordinary skill in the art will recognize that therefractive indices of an optical fiber are equivalent to those of theoptical-fiber preform from which the optical fiber is drawn.Furthermore, the radii of the core and cladding layers within an opticalfiber are determined by the radii of the core and cladding layers withinthe optical-fiber preform from which the optical fiber is drawn. Thus,reference to an optical fiber's refractive-index profile can be readilyextrapolated to the corresponding optical-fiber preform. That said,those having ordinary skill in the art will appreciate that the drawingprocess might cause an optical fiber's refractive index to deviateslightly from its corresponding optical-fiber preform.

The refractive-index profile of FIG. 1 depicts a central core havingradius R_(co) and a refractive index Dn_(co), which corresponds to therefractive index of pure silica. An inner depressed cladding having anouter radius R_(cl1) and a refractive index Dn_(cl1) surrounds thecentral core. The inner cladding depicted in FIG. 1 is depressed,because it has a refractive index that is less than the refractive indexof the outer cladding Dn_(out). The outer cladding is obtained byovercladding (e.g., by sleeving the primary preform). The outer claddingis generally formed of pure-silica glass and, therefore, hassubstantially the same refractive index as the central core in a PSCF.Typically, the outer cladding is formed from a substrate tube used tomake the primary preform and/or from the overcladding used to reach thedesired diameter ratio.

In the refractive-index profile depicted in FIG. 1, the fundamental modeLP01 is not completely guided and thus has additional losses, calledleakage. To minimize these leakage losses, the percentage of energypropagating in the outer pure-silica cladding should be reduced. Theratio between the outer radius of the fluorine-doped inner cladding andthe radius of the core (R_(cl1)/R_(co)) should therefore be sufficientlyhigh. In other words, the inner depressed cladding should be extended atleast as far as a critical radius whose value is dependent on the coreradius and the refractive-index difference between the core refractiveindex Dn_(co) and the refractive index of the inner cladding Dn_(cl1).For a standard SMF compliant with the ITU-T G.652 recommendations, it isthought that a ratio of eight or more between the outer radius of theinner depressed cladding and the radius of the core (i.e.,R_(cl1)/R_(co)>8) ensures good confinement of the optical signal in thecentral core and an acceptable level of leakage losses.

MCVD, FCVD, and PCVD techniques are satisfactory to obtain a goodquality central core and a large, highly depressed inner cladding. Thesetechniques, however, are costly whenever large capacity preforms aresought. The capacity of an optical-fiber preform is defined as thelength of optical fiber that can be drawn from that preform. The greaterthe diameter of the preform, the greater its capacity. To reducemanufacturing costs, it is desirable to provide long lengths of opticalfiber from one optical-fiber preform. It is therefore desirable tofabricate large-diameter preforms while complying with dimensionalconstraints relating to the diameter of the central core and thediameter of the optical cladding. After overcladding, the final preform(i.e., the optical-fiber preform) must present the same ratio of corediameter to cladding diameter as is to be achieved in the optical fiberdrawn therefrom.

U.S. Patent Application Publication No. 2008/0031582 and U.S. Pat. No.5,044,724, each of which is hereby incorporated by reference in itsentirety, disclose using a fluorine-doped deposition tube to make theprimary preform. This solution helps to minimize the quantity offluorine-doped layers deposited inside the tube. InternationalPublication No. 2010/003856 and its counterpart U.S. Patent PublicationNo. 2011/0100062, each of which is hereby incorporated by reference inits entirety, disclose the fabrication of fluorine-doped tubes by POD(Plasma Outside Deposition) or OVD.

When a fluorine-doped deposition tube is used, the depressed cladding ofthe primary preform is composed of the inner deposited cladding and thedeposition tube itself. The ratio between the outer radius of thedepressed cladding and the radius of the core can thereby be increasedwhile limiting the quantity of deposition inside the tube. Thissolution, however, is not practical for very thick tubes because thedeposition conditions change when a fluorine-doped tube is used insteadof a pure-silica tube, ultimately limiting the reduction of the quantitydeposited inside the tube.

U.S. Patent Application Publication No. 2007/0003198, which is herebyincorporated by reference in its entirety, discloses a hybrid process inwhich a rod used to form a germanium-doped core region is made by VAD orOVD and a cladding region is deposited inside a tube by MCVD. The corerod and the MCVD cladding tube are then assembled using a rod-in-tubetechnique. The optical fibers disclosed in this publication, however, donot have pure-silica cores or depressed claddings. As a result, thesefibers do not face the same issues faced by PSCFs, namely achieving lowattenuations at both 1383 nanometers and 1550 nanometers.

U.S. Patent Application Publication No. 2003/0063878, which is herebyincorporated by reference in its entirety, discloses a method formanufacturing a large preform. This publication discloses that the coreand inner claddings are deposited by CVD in a deposition tube that isafterwards completely removed. The outer cladding is deposited byoutside deposition or rod-in-tube methods. This publication aims tocontrol attenuation at 1550 nanometers for non-zero dispersion-shiftedfibers or dispersion-compensating fibers.

U.S. Patent Application Publication No. 2004/0159124, which is herebyincorporated by reference in its entirety, discloses a method tomanufacture large preforms. The core is deposited by MCVD in adeposition tube that is afterwards completely removed. A doped overcladtube can then be used to extend the depressed region.

None of the foregoing publications, however, discloses a PSCF or aslightly updoped-core fiber having controlled leakage losses and reducedattenuation at both 1383 nanometers and 1550 nanometers.

SUMMARY

The present invention facilitates the manufacturing of a large-capacityoptical-fiber preform while ensuring the optical quality of the drawnoptical fiber (e.g., pure-silica-core fibers and slightly updoped-corefibers). Therefore, optical fibers in accordance with the presentinvention typically have low attenuations at both 1383 nanometers and1550 nanometers.

In one aspect, the present invention embraces a single-mode opticalfiber that includes a central core surrounded by an outer cladding(i.e., an outer optical cladding). The optical fiber includes at leastfirst and second depressed claddings (i.e., inner depressed claddings)positioned between the central core and the outer cladding. The centralcore typically has a radius (R_(co)) of between about 3.5 microns (μm)and 5.5 microns and a refractive-index difference with the outercladding of between about −1×10⁻³ and 3×10⁻³ (e.g., between 0 and3×10⁻³). The first depressed cladding typically has an outer radius(R_(cl1)) of between about 9 microns and 15 microns and arefractive-index difference with the outer cladding of between about−5.5×10⁻³ and −2.5×10⁻³. The second depressed cladding typically has anouter radius (R_(cl2)) of between about 38 microns and 42 microns and arefractive-index difference with the first depressed cladding of betweenabout −0.5×10⁻³ and 0.5×10⁻³. The outer cladding typically has an outerradius (R_(oc)) of between about 61.5 microns and 63.5 microns.

In one embodiment, the optical fiber includes a third depressed claddingpositioned between the first and second depressed claddings. The thirddepressed cladding typically has (i) an outer radius (R_(cl3)) ofbetween about 15 microns and 25 microns, (ii) a refractive-indexdifference with the first depressed cladding of between about −0.5×10⁻³and 0.5×10⁻³, and (iii) a refractive-index difference with the seconddepressed cladding of between about −0.5×10⁻³ and 0.5×10⁻³.

The central core and the outer cladding are each typically made ofundoped silica (i.e., pure silica). Each of the depressed claddings istypically made of fluorine-doped silica.

The present optical fibers possess excellent performancecharacteristics. For example, at a wavelength of 1383 nanometers, theoptical fiber typically has attenuation of less than about 0.35 dB/km,more typically less than about 0.32 dB/km.

At a wavelength of 1550 nanometers, the optical fiber typically hasattenuation of less than about 0.18 dB/km, leakage losses of less thanabout 0.01 dB/km, and macrobending losses of less than about 1 dB/turnaround a bend radius (R_(c)) of 10 millimeters.

Likewise, at a wavelength of 1625 nanometers, the optical fibertypically has macrobending losses of less than about 0.05 dB/100 turnsaround a bend radius (R_(c)) of 30 millimeters and less than about 3dB/turn around a bend radius (R_(c)) of 10 millimeters.

In another aspect, the present invention embraces methods formanufacturing an optical-fiber preform and, optionally, a single-modeoptical fiber.

According to one embodiment, a method for manufacturing an optical-fiberpreform includes the steps of (i) depositing layers inside a depositiontube to form a central core and a first depressed cladding, (ii)completely removing the deposition tube, (iii) providing a seconddepressed cladding around the first depressed cladding, and (iv)providing an outer cladding around second depressed cladding, therebyforming the optical preform. Collapsing of the deposited central coreand deposited first depressed cladding typically occurs after theselayers are deposited inside the deposition tube but before thedeposition tube has been removed. After the final optical preform hasbeen formed, a single-mode optical fiber may be drawn.

The deposition tube may be made of undoped quartz. The deposition tubemay be removed by chemical etching, flame polishing, or using amechanical technique such as grinding or polishing. A combination ofthese techniques may also be used to remove the deposition tube.

According to another embodiment, a method for manufacturing anoptical-fiber preform includes the steps of (i) depositing layers insidea fluorine-doped-silica deposition tube to form a central core and afirst depressed cladding, (ii) providing a second depressed claddingaround the deposition tube, the deposition tube forming a thirddepressed cladding, and (iii) providing an outer cladding, therebyforming the optical preform. Collapsing typically occurs after thecentral-core and first-cladding layers are deposited inside thefluorine-doped-silica deposition tube. The second depressed cladding maybe made by overcladding with doped silica (e.g., by sleeving with adoped tube or by outside deposition with doped silica). After the finaloptical preform has been formed, a single-mode optical fiber may bedrawn.

According to yet another embodiment, a method for manufacturing anoptical-fiber preform includes the steps of (i) providing a core rod byoutside deposition, (ii) providing at least two successive depressedcladdings, and (iii) providing an outer cladding, thereby forming theoptical preform. Each of the successive depressed claddings may be madeby overcladding with doped silica (e.g., by sleeving with a doped tubeor by outside deposition with doped silica). Thereafter, a single-modeoptical fiber may be drawn from the resulting optical preform.

The foregoing processes can yield exemplary optical-fiber preforms thatmay be described by various cross-sectional-area ratios. Suchoptical-fiber preforms are within the scope of the present invention.

In this regard, the ratio of the cross-sectional area of the centralcore to the annular, cross-sectional area of the outer cladding istypically between about 0.0047 and 0.015.

The ratio of the annular, cross-sectional area of the first depressedcladding to the annular, cross-sectional area of the outer cladding istypically between about 0.019 and 0.11.

If a third depressed cladding is absent, the ratio of the annular,cross-sectional area of the second depressed cladding to the annular,cross-sectional area of the outer cladding is typically between about0.47 and 0.84.

On the other hand, if a third depressed cladding is present, the ratioof the annular, cross-sectional area of the second depressed cladding tothe annular, cross-sectional area of the outer cladding is typicallybetween about 0.31 and 0.77, and the ratio of the cross-sectional areaof the third depressed cladding to the cross-sectional area of the outercladding is typically less than about 0.27.

As will be understood by those having ordinary skill in the art, a drawnoptical fiber will have substantially the same cross-sectional-arearatios as the preform from which it is drawn.

The foregoing illustrative summary, as well as other exemplaryobjectives and/or advantages of the invention, and the manner in whichthe same are accomplished, are further explained within the followingdetailed description and its accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts a set refractive-index profile of acomparative pure-silica-core fiber.

FIG. 2 schematically depicts a set refractive-index profile of anoptical fiber according to an embodiment of the present invention.

FIG. 3 schematically depicts a set refractive-index profile of anoptical fiber according to another embodiment of the present invention.

FIG. 4 schematically depicts a set refractive-index profile of anoptical fiber according to yet another embodiment of the presentinvention.

DETAILED DESCRIPTION

The present invention embraces single-mode optical fibers that have lowtransmission losses and that can be manufactured at reduced cost withoutdeteriorating the propagation characteristics of the optical fibers.

In one aspect, the present invention embraces an optical fiber having anundoped or a slightly updoped silica core. The present optical-fiberdesigns help to limit attenuation, particularly attenuation at 1550nanometers that can occur when the central core is doped with germanium.Several depressed claddings surround the central core. Providingsuccessive depressed claddings makes it possible to manufacture verylarge preforms at reduced costs. Moreover, the design of the depressedcladdings can help to minimize the leakage losses of the fundamentalLP01 mode while keeping the leakage losses of the higher-order LP11 modesufficiently high to ensure a cable cut-off wavelength (λ_(CC)) thatcomplies with the ITU-T G.652 recommendations (i.e., 1260 nanometers orless).

In another aspect, the present invention embraces methods formanufacturing an optical-fiber preform and a corresponding single-modeoptical fiber.

In one embodiment, the central core and an inner depressed cladding areformed by Chemical Vapor Deposition (CVD) inside a deposition tube(e.g., a glass substrate tube). This limits attenuation, notablyattenuation at 1383 nanometers caused by OH-peak. Typically, thedeposited central core, the deposited inner depressed cladding, and thedeposition tube are collapsed, after which the deposition tube isremoved completely. An outer depressed cladding (e.g., a second innerdepressed cladding) is produced with a down-doped sleeving tube or withan outside deposition technique to extend the depressed region. Thisprocedure reduces the width of the depressed inner cladding that isdeposited inside the deposition tube and thus places the deposition tubemuch closer to the central core. Accordingly, this technique yieldslarger-capacity preforms and lowers manufacturing costs.

In another embodiment, the central core and an inner depressed claddingare formed by CVD inside a down-doped deposition tube. This likewiselimits attenuation, notably the attenuation at 1383 nanometers due tothe OH-peak. The depressed cladding is then extended with a down-dopedsleeving tube or with an outside deposition technique (e.g., OVD).Collapsing typically occurs after the central core and inner depressedcladding have been deposited inside the down-doped deposition tube. Inthis embodiment, the depressed cladding is composed of three differentregions: (i) a region made with CVD inside the down-doped depositiontube, (ii) a region formed by the down-doped deposition tube itself, and(iii) a region formed by a down-doped sleeving tube or made by anoutside deposition technique. This procedure reduces the width of thedepressed inner cladding that is deposited inside the down-dopeddeposition tube and increases the capacity of the final preform,yielding lower manufacturing costs.

In yet another embodiment, the central core is formed by Outside VaporDeposition (OVD) or Vapor Axial Deposition (VAD) to obtain a core rod.First, second, and optionally third depressed claddings may be obtainedby doped tubes sleeved over the core rod. Alternatively, the depressedcladdings may be obtained with an outside deposition technique, such asOVD. Accordingly, the attenuation issues experienced at 1383 nanometers,which are thought to be linked to the use of VAD or OVD techniques toform fluorine-doped claddings, can be avoided.

FIGS. 2 and 3 depict set refractive-index profiles of respective opticalfibers in accordance with the present invention. A single-mode fiber inaccordance with the present invention typically includes a central coresurrounded by an outer cladding (i.e., an outer optical cladding). Afirst depressed cladding and a second depressed cladding are positionedbetween the central core and the outer cladding. The outer cladding,which has a refractive index Dn_(out), is typically formed from undopedsilica or slightly doped silica. The central core has a radius (R_(co))of typically between about 3.5 microns and 5.5 microns and arefractive-index difference with the outer cladding (Dn_(co)−Dn_(out))of typically between about −3×10⁻³ and 3×10⁻³ (e.g., between about−1×10⁻³ and 3×10⁻³), more typically between about 0 and 3×10⁻³. Evenmore typically, the central core has a refractive-index difference withthe outer cladding of between about 0.5×10⁻³ and 2.5×10⁻³. The slightdoping, if any, of the central core facilitates limited attenuation at1550 nanometers.

As depicted in FIG. 2, the core may have substantially the samerefractive index as the outer cladding. In this case, the central coreand/or the outer cladding may be made of undoped silica, slightly dopedsilica, or co-doped silica.

As depicted in FIG. 3, the central core may have a refractive index thatis slightly greater than the refractive index of the outer cladding. Inthis case, the central core may be made of slightly doped silica orco-doped silica, and the outer cladding may be made of undoped silica,thereby helping to minimize costs.

The first depressed cladding typically has an outer radius (R_(cl1)) ofbetween about 9 microns and 15 microns and a refractive-index difference(Dn_(cl1)−Dn_(out)) with the outer cladding of between about −5.5×10⁻³and −2.5×10⁻³. As compared with the inner depressed cladding depicted inFIG. 1, the first depressed cladding's relatively small radiusfacilitates limited deposition by CVD and thus reduced deposition costs.If the depressed region includes only two depressed claddings (e.g., asillustrated in FIGS. 2-3), then the second depressed cladding typicallyhas an outer radius (R_(cl2)) of between about 38 microns and 42 micronsand a refractive-index difference with the outer cladding(Dn_(cl2)−Dn_(out)) such that the refractive-index difference betweenthe first depressed cladding and the second depressed cladding(Dn_(cl2)−Dn_(out)) is between about −0.5×10⁻³ and 0.5×10⁻³.

FIG. 4 illustrates the set refractive-index profile of an optical fiberaccording to another embodiment of the present invention. In thisembodiment, a third depressed cladding is positioned between the firstand second depressed claddings. The third depressed cladding typicallyhas an outer radius (R_(cl3)) of between about 15 microns and 25microns. In this embodiment, the first depressed cladding typically hasan outer radius (R_(cl1)) of between about 9 microns and 15 microns, andthe second depressed cladding has an outer radius (R_(cl2)) of betweenabout 38 microns and 42 microns. The third depressed cladding typicallyhas a refractive-index difference (Dn_(cl3)−Dn_(out)) with the outercladding such that its index difference with both the first depressedcladding (Dn_(cl3)−Dn_(out)) and second depressed cladding(Dn_(cl3)−Dn_(cl2)) is between about −0.5×10⁻³ and 0.5×10⁻³.

The second and third depressed claddings help to limit leakage lossesand ensure that the cut-off wavelength is compliant with the ITU-T G.652recommendations.

The outer cladding is obtained by overcladding with a silica-basedmaterial. For example, the outer cladding may be obtained by sleevingwith a quartz tube or by employing any of the aforementioned outsidedeposition techniques. By way of further example, advanced plasma andvapor deposition (APVD) may be used to make the outer cladding.

The drawn optical fiber's outer cladding has an outer radius R_(out) ofabout 62.5 microns ±1 micron.

In one embodiment of the present invention, optical preformscorresponding to optical fibers having two depressed claddings (e.g., asillustrated in FIGS. 2-3) may be manufactured as follows.

The central core and the first depressed cladding may be obtained bydeposition inside a deposition tube. The second depressed cladding isobtained by sleeving with a down-doped tube or by using outsidedeposition techniques. The viscosity and/or refractive index of thefirst depressed cladding may differ from the viscosity and/or refractiveindex of the second depressed cladding.

More specifically, a deposition tube is provided, after which a centralcore and first depressed cladding are deposited by CVD inside thedeposition tube. After the central core and the first depressed claddinghave been deposited, the deposition tube is removed. Therefore, it ispossible to employ an undoped quartz tube as the deposition tube.Notably, the central core and the first depressed cladding may bedeposited by Plasma Chemical Vapor Deposition (PCVD), Modified ChemicalVapor Deposition (MCVD), or Furnace Chemical Vapor Deposition (FCVD).The deposition tube may be removed by chemical etching and/or mechanicalpolishing after deposition is complete.

Once the deposition tube is removed, the central core and the firstdepressed cladding may be sleeved with a doped tube, such as afluorine-doped tube. Fluorine-doped tubes are described, for example, inInternational Publication No. 2010/003856, U.S. Patent ApplicationPublication No. 2008/0031582, and U.S. Pat. No. 5,044,724, each of whichis hereby incorporated by reference in its entirety.

Sleeving with a doped tube forms the second depressed cladding, therebyextending the depressed region of the preform. In other words, typicallyonly the first depressed cladding, which is the portion of the depressedregion closest to the central core, is formed by CVD, whereas the seconddepressed cladding is typically added by sleeving. Because the quantityof CVD is limited, the cost of forming the preform is reduced even whileobtaining a large depressed region.

Finally, the preform is overcladded with silica-based material (e.g.,sleeved with a quartz tube) to reach the desired diameter ratio.

In another embodiment of the present invention, optical preformscorresponding to optical fibers having three depressed claddings (e.g.,as illustrated in FIG. 4) may be manufactured as follows.

The central core and the first depressed cladding may be obtained bydeposition inside a down-doped deposition tube, which constitutes thethird depressed cladding. The second depressed cladding is obtained bysleeving with a down-doped tube or by using outside depositiontechniques. The first depressed cladding may be deposited by CVD (e.g.,PCVD, MCVD, or FCVD). Because the quantity of CVD is limited, the costof forming the preform is reduced even while obtaining a large depressedregion.

Finally, the preform is overcladded with silica-based material (e.g.,sleeved with a quartz tube) to reach the desired diameter ratio.

In yet another embodiment of the present invention, optical preformscorresponding to optical fibers having two or three depressed claddingsmay be manufactured using outside deposition techniques and/or dopedtubes to obtain the first, second, and third depressed claddings. Theviscosity and/or refractive index of each depressed cladding may differfrom the viscosity and/or refractive index of the other depressedcladdings.

More specifically, a core rod may be obtained by OVD or VAD. Eachdepressed cladding may be obtained by any outside deposition techniqueor by sleeving with a doped tube. The outer cladding may be obtained byovercladding with a silica-based material, such as with an outsidedeposition technique (e.g., APVD) or by sleeving with a quartz tube, toreach the desired diameter ratio.

The optical preforms in accordance with the present invention may bedescribed by various ratios of cross-sectional areas. As will beunderstood by those having ordinary skill in the art, the followingequations use R to refer to the outer radius of the respectiveoptical-preform component.

The ratio of the cross-sectional area of the central core to theannular, cross-sectional area of the outer cladding is typically betweenabout 0.0047 and 0.015. In this regard, the ratio of the cross-sectionalarea of the central core to the cross-sectional area of the outercladding is equal to:

$\frac{R_{core}^{2}}{R_{{outer}\mspace{14mu} {cladding}}^{2} - R_{{second}\mspace{14mu} {depressed}\mspace{14mu} {cladding}}^{2}}.$

The ratio of the annular, cross-sectional area of the first depressedcladding to the annular, cross-sectional area of the outer cladding istypically between about 0.019 and 0.11. In this regard, the ratio of thecross-sectional area of the first depressed cladding to thecross-sectional area of the outer cladding is equal to:

$\frac{R_{{first}\mspace{14mu} {depressed}\mspace{14mu} {cladding}}^{2} - R_{{core}\;}^{2}}{R_{{outer}\mspace{14mu} {cladding}}^{2} - R_{{second}\mspace{14mu} {depressed}\mspace{14mu} {cladding}}^{2}}.$

If a third depressed cladding is absent, the ratio of the annular,cross-sectional area of the second depressed cladding to the annular,cross-sectional area of the outer cladding is typically between about0.47 and 0.84. In this scenario, the ratio of the cross-sectional areaof the second depressed cladding to the cross-sectional area of theouter cladding is equal to:

$\frac{R_{{second}\mspace{14mu} {depressed}\mspace{14mu} {cladding}}^{2} - R_{{first}\mspace{14mu} {depressed}\mspace{14mu} {cladding}}^{2}}{R_{{outer}\mspace{14mu} {cladding}}^{2} - R_{{{second}\mspace{14mu} {depressed}\mspace{14mu} {cladding}}\;}^{2}}.$

If a third depressed cladding is present, the ratio of the annular,cross-sectional area of the second depressed cladding to the annular,cross-sectional area of the outer cladding is typically between about0.31 and 0.77, and the ratio of the cross-sectional area of the thirddepressed cladding to the cross-sectional area of the outer cladding istypically less than about 0.27. In this scenario, the ratio of thecross-sectional area of the second depressed cladding to thecross-sectional area of the outer cladding is equal to:

$\frac{R_{{second}\mspace{14mu} {depressed}\mspace{14mu} {cladding}}^{2} - R_{{third}\mspace{14mu} {depressed}\mspace{14mu} {cladding}}^{2}}{R_{{outer}\mspace{14mu} {cladding}}^{2} - R_{{second}\mspace{14mu} {depressed}\mspace{14mu} {cladding}}^{2}},$

Similarly, in this scenario, the ratio of the annular, cross-sectionalarea of the third depressed cladding to the annular, cross-sectionalarea of the outer cladding is equal to:

$\frac{R_{{third}\mspace{14mu} {depressed}\mspace{14mu} {cladding}}^{2} - R_{{first}\mspace{14mu} {depressed}\mspace{14mu} {cladding}}^{2}}{R_{{outer}\mspace{14mu} {cladding}}^{2} - R_{{second}\mspace{14mu} {depressed}\mspace{14mu} {cladding}}^{2}}.$

Tables I-III provide prophetic, computer-modeling data with respect tocomparative and exemplary optical fibers.

Table I (below) depicts exemplary optical-fiber profiles (Ex. 1-5).Table I also depicts three comparative optical-fiber profiles (Comp. Ex.1-3). The refractive-index profile of Comparative Example 1 is depictedin FIG. 1.

TABLE I Dn_(co)-Dn_(out) Dn_(cl1)-Dn_(out) Dn_(cl3)-Dn_(out)Dn_(cl2)-Dn_(out) R_(co) R_(cl1) R_(cl3) R_(cl2) R_(out) (10⁻³) (10⁻³)(10⁻³) (10⁻³) (μm) (μm) (μm) (μm) (μm) (@633 nm) (@633 nm) (@633 nm)(@633 nm) Comp. 4.35 41.00 — — 62.50 0.0 −5.2 — — Ex. 1 Ex. 1 4.35 15.00— 41.00 62.50 0.5 −4.7 — −4.5 Ex. 2 4.35 13.80 — 41.00 62.50 2.2 −3.0 —−3.4 Comp. 4.35 13.80 — 41.00 62.50 2.2 −3.0 — −4.0 Ex. 2 Ex. 3 4.359.50 — 39.00 62.50 2.2 −3.0 — −3.0 Comp. 4.35 9.50 — 39.00 62.50 2.2−3.0 — −2.4 Ex. 3 Ex. 4 4.50 15.00 — 40.00 62.50 1.0 −4.5 — −4.5 Ex. 54.35 15.00 22.00 41.00 62.50 0.5 −4.7 −4.6 −4.5

Table II (below) depicts optical characteristics of Comparative Examples1-3 and Examples 1-5. In this regard, Table II discloses cable cut-offwavelength (λ_(CC)), mode field diameter (2W₀₂), chromatic dispersion(D), chromatic dispersion slope (S), zero-dispersion wavelength (ZDW),zero-dispersion slope (ZDS), and leakage losses.

TABLE II leakage 2W₀₂ 2W₀₂ D S losses λ_(cc) (@1310 nm) (@1550 nm)(@1550 nm) (@1550 nm) ZDW ZDS (@1550 nm) (nm) (μm²) (μm²) (ps/nm · km)(ps/nm² · km) (nm) (ps/nm² · km) (dB/km) Comp. 1240 9.2 10.3 16.3 0.0561315 0.086 0.001 Ex. 1 Ex. 1 <1260 9.1 10.3 16.2 0.055 1306 0.083 0.006Ex. 2 <1260 9.1 10.3 16.4 0.056 1309 0.084 0.001 Comp. >1400 9.1 10.316.4 0.057 1309 0.084 0.000 Ex. 2 Ex. 3 <1260 9.1 10.3 16.3 0.056 13090.084 0.009 Comp. <1260 9.1 10.3 15.6 0.053 1312 0.082 >0.1 Ex. 3 Ex. 4<1260 9.1 10.2 17.1 0.056 1300 0.086 0.001 Ex. 5 <1260 9.1 10.3 16.20.055 1306 0.083 0.005

Table III (below) depicts macrobending losses around the specified bendradius (R_(c)) and attenuation of Comparative Examples 1-3 and Examples1-5.

TABLE III Macrobending losses (@1625 nm) (@1550 nm) (@1625 nm) (R_(c) =Attenuation (R_(c) = (R_(c) = 30 mm) (@1550 (@1383 10 mm) 10 mm) (dB/nm) nm) (dB/turn) (dB/turn) 100 turns) (dB/km) (dB/km) Comp. 0.2 0.7<0.05 <0.18 <0.32 Ex. 1 Ex. 1 0.8 2.6 <0.05 <0.18 <0.32 Ex. 2 0.1 0.3<0.01 <0.18 <0.35 Comp. <0.05 <0.1 <0.01 <0.18 <0.35 Ex. 2 Ex. 3 0.4 1.1<0.05 <0.18 <0.35 Comp. >1 >5 >1 >0.19 >0.35 Ex. 3 Ex. 4 0.09 0.3 <0.01<0.18 <0.32 Ex. 5 0.8 2.6 <0.05 <0.18 <0.32

Comparative Examples 2-3 are outside the scope of the present invention,because the absolute value of the refractive-index difference betweenthe first and second depressed claddings (Dn_(cl2)−Dn_(cl1)) is too high(i.e., more than ±0.5×10⁻³). As depicted in Table II, ComparativeExample 2 has a cut-off wavelength that does not comply with the ITU-TG.652 recommendations. As depicted in Table II and Table III,Comparative Example 3 has high leakage losses, high bending losses, andhigh attenuation.

As noted, optical fibers in accordance with the present inventiontypically comply with the ITU-T G.652 recommendations. Examples 1-5, forinstance, satisfy the ITU-T G.652 recommendations. As depicted in TableII, optical fibers in accordance with the present invention typicallyhave low leakage losses. As will be understood by those having ordinaryskill in the art, leakage losses can be derived from spectralattenuation measurements.

As depicted in Table III, optical fibers in accordance with the presentinvention typically have low macrobending losses. Table III alsodemonstrates that the present optical fibers typically have lowattenuation, particularly low attenuation at 1383 nanometers and 1550nanometers. In this regard, the present optical fibers typically haveattenuation of less than about 0.18 dB/km at a wavelength of 1550nanometers. The present optical fibers also typically have attenuationof less than about 0.35 dB/km (e.g., less than 0.32 dB/km) at awavelength of 1383 nanometers. The use of a quartz deposition tube todeposit a slightly doped silica core makes it possible to reduce theOH-peak. In addition, the removal of the deposition tube does notjeopardize the optical characteristics of the resulting optical fiber.

Optical fibers in accordance with the present invention can bemanufactured from a large-capacity preform. Examples 1, 4, and 5 achievefinal preforms having a first depressed cladding radius R_(cl1) of 10millimeters and an outside diameter of 83.3 millimeters by using adeposition tube with a cross-sectional area of 180 mm². Example 2achieves a final preform having an outside diameter of 90.6 millimeters,and Example 3 achieves a final preform having an outside diameter of131.6 millimeters.

By way of comparison, Comparative Example 1 retains the deposition tubeas part of the outer cladding to yield a final preform with an outsidediameter of 30.5 millimeters.

The optical fibers in accordance with the present invention cantherefore be manufactured at reduced cost. Indeed, the width of thefirst depressed cladding deposited inside the deposition tube can bemuch smaller if the present refractive-index profiles are employed.

The present optical fibers may facilitate the reduction in overalloptical-fiber diameter. As will be appreciated by those having ordinaryskill in the art, a reduced-diameter optical fiber is cost-effective,requiring less raw material. Moreover, a reduced-diameter optical fiberrequires less deployment space (e.g., within a buffer tube and/or fiberoptic cable), thereby facilitating increased fiber count and/or reducedcable size.

Those having ordinary skill in the art will recognize that an opticalfiber with a primary coating (and an optional secondary coating and/orink layer) typically has an outer diameter of between about 235 micronsand about 265 microns (μm). The component glass fiber itself (i.e., theglass core and surrounding cladding layers) typically has a diameter ofabout 125 microns, such that the total coating thickness is typicallybetween about 55 microns and 70 microns.

With respect to the present optical fiber, the component glass fibertypically has an outer diameter of about 125 microns. With respect tothe optical fiber's surrounding coating layers, the primary coatingtypically has an outer diameter of between about 175 microns and about195 microns (i.e., a primary coating thickness of between about 25microns and 35 microns), and the secondary coating typically has anouter diameter of between about 235 microns and about 265 microns (i.e.,a secondary coating thickness of between about 20 microns and 45microns). Optionally, the present optical fiber may include an outermostink layer, which is typically between two and ten microns in thickness.

In one alternative embodiment, an optical fiber may possess a reduceddiameter (e.g., an outermost diameter between about 150 microns and 230microns). In this alternative optical fiber configuration, the thicknessof the primary coating and/or secondary coating is reduced, while thediameter of the component glass fiber is maintained at about 125microns. (Those having ordinary skill in the art will appreciate that,unless otherwise specified, diameter measurements refer to outerdiameters.)

By way of illustration, in such exemplary embodiments, the primarycoating layer may have an outer diameter of between about 135 micronsand about 175 microns (e.g., about 160 microns), typically less than 165microns (e.g., between about 135 microns and 150 microns), and usuallymore than 140 microns (e.g., between about 145 microns and 155 microns,such as about 150 microns).

Moreover, in such exemplary embodiments, the secondary coating layer mayhave an outer diameter of between about 150 microns and about 230microns (e.g., more than about 165 microns, such as 190-210 microns orso), typically between about 180 microns and 200 microns. In otherwords, the total diameter of the optical fiber is reduced to less thanabout 230 microns (e.g., between about 195 microns and 205 microns, andespecially about 200 microns). By way of further illustration, anoptical fiber may employ a secondary coating of about 197 microns at atolerance of +/−5 microns (i.e., a secondary-coating outer diameter ofbetween 192 microns to 202 microns). Typically, the secondary coatingwill retain a thickness of at least about 10 microns (e.g., an opticalfiber having a reduced thickness secondary coating of between 15 micronsand 25 microns).

In another alternative embodiment, the outer diameter of the componentglass fiber may be reduced to less than 125 microns (e.g., between about60 microns and 120 microns), perhaps between about 70 microns and 115microns (e.g., about 80-110 microns). This may be achieved, forinstance, by reducing the thickness of one or more cladding layers. Ascompared with the prior alternative embodiment, (i) the total diameterof the optical fiber may be reduced (i.e., the thickness of the primaryand secondary coatings are maintained in accordance with the prioralternative embodiment) or (ii) the respective thicknesses of theprimary and/or secondary coatings may be increased relative to the prioralternative embodiment (e.g., such that the total diameter of theoptical fiber might be maintained).

By way of illustration, with respect to the former, a component glassfiber having a diameter of between about 90 and 100 microns might becombined with a primary coating layer having an outer diameter ofbetween about 110 microns and 150 microns (e.g., about 125 microns) anda secondary coating layer having an outer diameter of between about 130microns and 190 microns (e.g., about 155 microns). With respect to thelatter, a component glass fiber having a diameter of between about 90and 100 microns might be combined with a primary coating layer having anouter diameter of between about 120 microns and 140 microns (e.g., about130 microns) and a secondary coating layer having an outer diameter ofbetween about 160 microns and 230 microns (e.g., about 195-200 microns).

Reducing the diameter of the component glass fiber might make theresulting optical fiber more susceptible to microbending attenuation.That said, the advantages of further reducing optical-fiber diametermight be worthwhile for some optical-fiber applications.

As noted, the present optical fibers may include one or more coatinglayers (e.g., a primary coating and a secondary coating). At least oneof the coating layers—typically the secondary coating—may be coloredand/or possess other markings to help identify individual fibers.Alternatively, a tertiary ink layer may surround the primary andsecondary coatings.

The present optical fibers may be deployed in various structures, suchas those exemplary structures disclosed hereinafter.

For example, one or more of the present optical fibers may be enclosedwithin a buffer tube. For instance, optical fiber may be deployed ineither a single-fiber loose buffer tube or a multi-fiber loose buffertube. With respect to the latter, multiple optical fibers may be bundledor stranded within a buffer tube or other structure. In this regard,within a multi-fiber loose buffer tube, fiber sub-bundles may beseparated with binders (e.g., each fiber sub-bundle is enveloped in abinder). Moreover, fan-out tubing may be installed at the termination ofsuch loose buffer tubes to directly terminate loose buffered opticalfibers with field-installed connectors.

In other embodiments, the buffer tube may tightly surround the outermostoptical fiber coating (i.e., tight buffered fiber) or otherwise surroundthe outermost optical-fiber coating or ink layer to provide an exemplaryradial clearance of between about 50 and 100 microns (i.e., a semi-tightbuffered fiber).

With respect to the former tight buffered fiber, the buffering may beformed by coating the optical fiber with a curable composition (e.g., aUV-curable material) or a thermoplastic material. The outer diameter oftight buffer tubes, regardless of whether the buffer tube is formed froma curable or non-curable material, is typically less than about 1,000microns (e.g., either about 500 microns or about 900 microns).

With respect to the latter semi-tight buffered fiber, a lubricant may beincluded between the optical fiber and the buffer tube (e.g., to providea gliding layer).

As will be known by those having ordinary skill in the art, an exemplarybuffer tube enclosing optical fibers as disclosed herein may be formedof polyolefins (e.g., polyethylene or polypropylene), includingfluorinated polyolefins, polyesters (e.g., polybutylene terephthalate),polyamides (e.g., nylon), as well as other polymeric materials andblends. In general, a buffer tube may be formed of one or more layers.The layers may be homogeneous or include mixtures or blends of variousmaterials within each layer.

In this context, the buffer tube may be extruded (e.g., an extrudedpolymeric material) or pultruded (e.g., a pultruded, fiber-reinforcedplastic). By way of example, the buffer tube may include a material toprovide high temperature and chemical resistance (e.g., an aromaticmaterial or polysulfone material).

Although buffer tubes typically have a circular cross section, buffertubes alternatively may have an irregular or non-circular shape (e.g.,an oval or a trapezoidal cross-section).

Alternatively, one or more of the present optical fibers may simply besurrounded by an outer protective sheath or encapsulated within a sealedmetal tube. In either structure, no intermediate buffer tube isnecessarily required.

Multiple optical fibers as disclosed herein may be sandwiched,encapsulated, and/or edge bonded to form an optical fiber ribbon.Optical fiber ribbons can be divisible into subunits (e.g., atwelve-fiber ribbon that is splittable into six-fiber subunits).Moreover, a plurality of such optical fiber ribbons may be aggregated toform a ribbon stack, which can have various sizes and shapes.

For example, it is possible to form a rectangular ribbon stack or aribbon stack in which the uppermost and lowermost optical fiber ribbonshave fewer optical fibers than those toward the center of the stack.This construction may be useful to increase the density of opticalelements (e.g., optical fibers) within the buffer tube and/or cable.

In general, it is desirable to increase the filling of transmissionelements in buffer tubes or cables, subject to other constraints (e.g.,cable or mid-span attenuation). The optical elements themselves may bedesigned for increased packing density. For example, the optical fibermay possess modified properties, such as improved refractive-indexprofile, core or cladding dimensions, or primary-coating thicknessand/or modulus, to improve microbending and macrobendingcharacteristics.

By way of example, a rectangular ribbon stack may be formed with orwithout a central twist (i.e., a “primary twist”). Those having ordinaryskill in the art will appreciate that a ribbon stack is typicallymanufactured with rotational twist to allow the tube or cable to bendwithout placing excessive mechanical stress on the optical fibers duringwinding, installation, and use. In a structural variation, a twisted (oruntwisted) rectangular ribbon stack may be further formed into acoil-like configuration (e.g., a helix) or a wave-like configuration(e.g., a sinusoid). In other words, the ribbon stack may possess regular“secondary” deformations.

As will be known to those having ordinary skill in the art, such opticalfiber ribbons may be positioned within a buffer tube or othersurrounding structure, such as a buffer-tube-free cable. Subject tocertain restraints (e.g., attenuation), it is desirable to increase thedensity of elements such as optical fibers or optical fiber ribbonswithin buffer tubes and/or optical fiber cables.

A plurality of buffer tubes containing optical fibers (e.g., loose orribbonized fibers) may be positioned externally adjacent to and strandedaround a central strength member. This stranding can be accomplishedhelically in one direction, known as “S” or “Z” stranding, or viaReverse Oscillated Lay stranding, known as “S-Z” stranding. Strandingabout the central strength member reduces optical fiber strain whencable strain occurs during installation and use.

Those having ordinary skill in the art will understand the benefit ofminimizing fiber strain for both tensile cable strain and longitudinalcompressive cable strain during installation or operating conditions.

With respect to tensile cable strain, which may occur duringinstallation, the cable will become longer while the optical fibers canmigrate closer to the cable's neutral axis to reduce, if not eliminate,the strain being translated to the optical fibers. With respect tolongitudinal compressive strain, which may occur at low operatingtemperatures due to shrinkage of the cable components, the opticalfibers will migrate farther away from the cable's neutral axis toreduce, if not eliminate, the compressive strain being translated to theoptical fibers.

In a variation, two or more substantially concentric layers of buffertubes may be positioned around a central strength member. In a furthervariation, multiple stranding elements (e.g., multiple buffer tubesstranded around a strength member) may themselves be stranded aroundeach other or around a primary central strength member.

Alternatively, a plurality of buffer tubes containing optical fibers(e.g., loose or ribbonized fibers) may be simply placed externallyadjacent to the central strength member (i.e., the buffer tubes are notintentionally stranded or arranged around the central strength member ina particular manner and run substantially parallel to the centralstrength member).

Alternatively still, the present optical fibers may be positioned withina central buffer tube (i.e., the central buffer tube cable has a centralbuffer tube rather than a central strength member). Such a centralbuffer tube cable may position strength members elsewhere. For instance,metallic or non-metallic (e.g., GRP) strength members may be positionedwithin the cable sheath itself, and/or one or more layers ofhigh-strength yarns (e.g., aramid or non-aramid yarns) may be positionedparallel to or wrapped (e.g., contrahelically) around the central buffertube (i.e., within the cable's interior space). As will be understood bythose having ordinary skill in the art, such strength yarns providetensile strength to fiber optic cables. Likewise, strength members canbe included within the buffer tube's casing.

Strength yarns may be coated with a lubricant (e.g., fluoropolymers),which may reduce unwanted attenuation in fiber optic cables (e.g.,rectangular, flat ribbon cables or round, loose tube cables) that aresubjected to relatively tight bends (i.e., a low bend radius). Moreover,the presence of a lubricant on strength yarns (e.g., aramid strengthyarns) may facilitate removal of the cable jacketing by reducingunwanted bonding between the strength yarns and the surrounding cablejacket.

In other embodiments, the optical fibers may be placed within a slottedcore cable. In a slotted core cable, optical fibers, individually or asa fiber ribbon, may be placed within pre-shaped helical grooves (i.e.,channels) on the surface of a central strength member, thereby forming aslotted core unit. The slotted core unit may be enclosed by a buffertube. One or more of such slotted core units may be placed within aslotted core cable. For example, a plurality of slotted core units maybe helically stranded around a central strength member.

Alternatively, the optical fibers may also be stranded in a maxitubecable design, whereby the optical fibers are stranded around themselveswithin a large multi-fiber loose buffer tube rather than around acentral strength member. In other words, the large multi-fiber loosebuffer tube is centrally positioned within the maxitube cable. Forexample, such maxitube cables may be deployed in optical ground wires(OPGW).

In another cabling embodiment, multiple buffer tubes may be strandedaround themselves without the presence of a central member. Thesestranded buffer tubes may be surrounded by a protective tube. Theprotective tube may serve as the outer casing of the fiber optic cableor may be further surrounded by an outer sheath. The protective tube mayeither tightly surround or loosely surround the stranded buffer tubes.

As will be known to those having ordinary skill in the art, additionalelements may be included within a cable core. For example, copper cablesor other active, transmission elements may be stranded or otherwisebundled within the cable sheath. Passive elements may also be placedwithin the cable core, such as between the interior walls of the buffertubes and the enclosed optical fibers. Alternatively and by way ofexample, passive elements may be placed outside the buffer tubes betweenthe respective exterior walls of the buffer tubes and the interior wallof the cable jacket, or within the interior space of a buffer-tube-freecable.

For example, yarns, nonwovens, fabrics (e.g., tapes), foams, or othermaterials containing water-swellable material and/or coated withwater-swellable materials (e.g., including super absorbent polymers(SAPs), such as SAP powder) may be employed to provide water blockingand/or to couple the optical fibers to the surrounding buffer tubeand/or cable jacketing (e.g., via adhesion, friction, and/orcompression). Exemplary water-swellable elements are disclosed incommonly assigned U.S. Pat. No. 7,515,795 for a Water-Swellable Tape,Adhesive-Backed for Coupling When Used Inside a Buffer Tube, which ishereby incorporated by reference in its entirety.

Moreover, an adhesive (e.g., a hot-melt adhesive or curable adhesive,such as a silicone acrylate cross-linked by exposure to actinicradiation) may be provided on one or more passive elements (e.g.,water-swellable material) to bond the elements to the buffer tube. Anadhesive material may also be used to bond the water-swellable elementto optical fibers within the buffer tube. Exemplary arrangements of suchelements are disclosed in commonly assigned U.S. Pat. No. 7,599,589 fora Gel-Free Buffer Tube with Adhesively Coupled Optical Element, which ishereby incorporated by reference in its entirety.

The buffer tubes (or buffer-tube-free cables) may also contain athixotropic composition (e.g., grease or grease-like gels) between theoptical fibers and the interior walls of the buffer tubes. For example,filling the free space inside a buffer tube with water-blocking,petroleum-based filling grease helps to block the ingress of water.Further, the thixotropic filling grease mechanically (i.e., viscously)couples the optical fibers to the surrounding buffer tube.

Such thixotropic filling greases are relatively heavy and messy, therebyhindering connection and splicing operations. Thus, the present opticalfibers may be deployed in dry cable structures (i.e., grease-free buffertubes).

Exemplary buffer tube structures that are free from thixotropic fillinggreases are disclosed in commonly assigned U.S. Pat. No. 7,724,998 for aCoupling Composition for Optical Fiber Cables (Parris et al.), which ishereby incorporated by reference in its entirety. Such buffer tubesemploy coupling compositions formed from a blend of high-molecularweight elastomeric polymers (e.g., about 35 weight percent or less) andoils (e.g., about 65 weight percent or more) that flow at lowtemperatures. Unlike thixotropic filling greases, the couplingcomposition (e.g., employed as a cohesive gel or foam) is typically dryand, therefore, less messy during splicing.

As will be understood by those having ordinary skill in the art, a cableenclosing optical fibers as disclosed herein may have a sheath formedfrom various materials in various designs. Cable sheathing may be formedfrom polymeric materials such as, for example, polyethylene,polypropylene, polyvinyl chloride (PVC), polyamides (e.g., nylon),polyester (e.g., PBT), fluorinated plastics (e.g., perfluorethylenepropylene, polyvinyl fluoride, or polyvinylidene difluoride), andethylene vinyl acetate. The sheath and/or buffer tube materials may alsocontain other additives, such as nucleating agents, flame-retardants,smoke-retardants, antioxidants, UV absorbers, and/or plasticizers.

The cable sheathing may be a single jacket formed from a dielectricmaterial (e.g., non-conducting polymers), with or without supplementalstructural components that may be used to improve the protection (e.g.,from rodents) and strength provided by the cable sheath. For example,one or more layers of metallic (e.g., steel) tape, along with one ormore dielectric jackets, may form the cable sheathing. Metallic orfiberglass reinforcing rods (e.g., GRP) may also be incorporated intothe sheath. In addition, aramid, fiberglass, or polyester yarns may beemployed under the various sheath materials (e.g., between the cablesheath and the cable core), and/or ripcords may be positioned, forexample, within the cable sheath.

Similar to buffer tubes, optical fiber cable sheaths typically have acircular cross section, but cable sheaths alternatively may have anirregular or non-circular shape (e.g., an oval, trapezoidal, or flatcross-section).

By way of example, the present optical fiber may be incorporated intosingle-fiber drop cables, such as those employed for Multiple DwellingUnit (MDU) applications. In such deployments, the cable jacketing mustexhibit crush resistance, abrasion resistance, puncture resistance,thermal stability, and fire resistance as required by building codes. Anexemplary material for such cable jackets is thermally stable,flame-retardant polyurethane (PUR), which mechanically protects theoptical fibers yet is sufficiently flexible to facilitate easy MDUinstallations. Alternatively, a flame-retardant polyolefin or polyvinylchloride sheath may be used.

In general, and as will be known to those having ordinary skill in theart, a strength member is typically in the form of a rod orbraided/helically wound wires or fibers, though other configurationswill be within the knowledge of those having ordinary skill in the art.

Optical fiber cables containing optical fibers as disclosed may bevariously deployed, including as drop cables, distribution cables,feeder cables, trunk cables, and stub cables, each of which may havevarying operational requirements (e.g., temperature range, crushresistance, UV resistance, and minimum bend radius).

Such optical fiber cables may be installed within ducts, microducts,plenums, or risers. By way of example, an optical fiber cable may beinstalled in an existing duct or microduct by pulling or blowing (e.g.,using compressed air). An exemplary cable installation method isdisclosed in commonly assigned U.S. Pat. No. 7,574,095 for aCommunication Cable Assembly and Installation Method, (Lock et al.), andU.S. Pat. No. 7,665,902 for a Modified Pre-Ferrulized CommunicationCable Assembly and Installation Method, (Griffioen et al.), each ofwhich is incorporated by reference in its entirety.

As noted, buffer tubes containing optical fibers (e.g., loose orribbonized fibers) may be stranded (e.g., around a central strengthmember). In such configurations, an optical fiber cable's protectiveouter sheath may have a textured outer surface that periodically varieslengthwise along the cable in a manner that replicates the strandedshape of the underlying buffer tubes. The textured profile of theprotective outer sheath can improve the blowing performance of theoptical fiber cable. The textured surface reduces the contact surfacebetween the cable and the duct or microduct and increases the frictionbetween the blowing medium (e.g., air) and the cable. The protectiveouter sheath may be made of a low coefficient-of-friction material,which can facilitate blown installation. Moreover, the protective outersheath can be provided with a lubricant to further facilitate blowninstallation.

In general, to achieve satisfactory long-distance blowing performance(e.g., between about 3,000 to 5,000 feet or more), the outer cablediameter of an optical fiber cable should be no more than about 70 to 80percent of the duct's or microduct's inner diameter.

Compressed air may also be used to install optical fibers in an airblown fiber system. In an air blown fiber system, a network of unfilledcables or microducts is installed prior to the installation of opticalfibers. Optical fibers may subsequently be blown into the installedcables as necessary to support the network's varying requirements.

Moreover, the optical fiber cables may be directly buried in the groundor, as an aerial cable, suspended from a pole or pylon. An aerial cablemay be self-supporting, or secured or lashed to a support (e.g.,messenger wire or another cable). Exemplary aerial fiber optic cablesinclude overhead ground wires (OPGW), all-dielectric self-supportingcables (ADSS), all dielectric lash cables (AD-Lash), and figure-eightcables, each of which is well understood by those having ordinary skillin the art. Figure-eight cables and other designs can be directly buriedor installed into ducts, and may optionally include a toning element,such as a metallic wire, so that they can be found with a metaldetector.

In addition, although the optical fibers may be further protected by anouter cable sheath, the optical fiber itself may be further reinforcedso that the optical fiber may be included within a breakout cable, whichallows for the individual routing of individual optical fibers.

To effectively employ the present optical fibers in a transmissionsystem, connections are required at various points in the network.Optical fiber connections are typically made by fusion splicing,mechanical splicing, or mechanical connectors.

The mating ends of connectors can be installed to the optical fiber endseither in the field (e.g., at the network location) or in a factoryprior to installation into the network. The ends of the connectors aremated in the field in order to connect the optical fibers together orconnect the optical fibers to the passive or active components. Forexample, certain optical fiber cable assemblies (e.g., furcationassemblies) can separate and convey individual optical fibers from amultiple optical fiber cable to connectors in a protective manner.

The deployment of such optical fiber cables may include supplementalequipment, which itself may employ the present optical fiber aspreviously disclosed. For instance, an amplifier may be included toimprove optical signals. Dispersion compensating modules may beinstalled to reduce the effects of chromatic dispersion and polarizationmode dispersion. Splice boxes, pedestals, and distribution frames, whichmay be protected by an enclosure, may likewise be included. Additionalelements include, for example, remote terminal switches, optical networkunits, optical splitters, and central office switches.

A cable containing the present optical fibers may be deployed for use ina communication system (e.g., networking or telecommunications). Acommunication system may include fiber optic cable architecture such asfiber-to-the-node (FTTN), fiber-to-the-telecommunications enclosure(FTTE), fiber-to-the-curb (FTTC), fiber-to-the-building (FTTB), andfiber-to-the-home (FTTH), as well as long-haul or metro architecture.Moreover, an optical module or a storage box that includes a housing mayreceive a wound portion of the optical fiber disclosed herein. By way ofexample, the optical fiber may be wound around a bend radius of lessthan about 15 millimeters (e.g., 10 millimeters or less, such as about 5millimeters) in the optical module or the storage box.

Moreover, present optical fibers may be used in other applications,including, without limitation, fiber optic sensors or illuminationapplications (e.g., lighting).

The present optical fibers may include Fiber Bragg Grating (FBG). Aswill be known by those having ordinary skill in the art, FBG is aperiodic or aperiodic variation in the refractive index of an opticalfiber core and/or cladding. This variation in the refractive indexresults in a range of wavelengths (e.g., a narrow range) being reflectedrather than transmitted, with maximum reflectivity occurring at theBragg wavelength.

Fiber Bragg Grating is commonly written into an optical fiber byexposing the optical fiber to an intense source of ultraviolet light(e.g., a UV laser). In this respect, UV photons may have enough energyto break molecular bonds within an optical fiber, which alters thestructure of the optical fiber, thereby increasing the optical fiber'srefractive index. Moreover, dopants (e.g., boron or germanium) and/orhydrogen loading can be employed to increase photosensitivity.

In order to expose a coated glass fiber to UV light for the creation ofFBG, the coating may be removed. Alternatively, coatings that aretransparent at the particular UV wavelengths (e.g., the UV wavelengthsemitted by a UV laser to write FBG) may be employed to render coatingremoval unnecessary. In addition, silicone, polyimide, acrylate, or PFCBcoatings, for instance, may be employed for high-temperatureapplications.

A particular FBG pattern may be created by employing (i) a photomaskplaced between the UV light source and the optical fiber, (ii)interference between multiple UV light beams, which interfere with eachother in accordance with the desired FBG pattern (e.g., a uniform,chirped, or titled pattern), or (iii) a narrow UV light beam forcreating individual variations. The FBG structure may have, for example,a uniform positive-only index change, a Gaussian-apodized index change,a raised-cosine-apodized index change, or a discrete phase-shift indexchange. Multiple FBG patterns may be combined on a single optical fiber.

Optical fibers having FBG may be employed in various sensingapplications (e.g., for detecting vibration, temperature, pressure,moisture, or movement). In this respect, changes in the optical fiber(e.g., a change in temperature) result in a shift in the Braggwavelength, which is measured by a sensor. FBG may be used to identify aparticular optical fiber (e.g., if the optical fiber is broken intopieces).

Fiber Bragg Grating may also be used in various active or passivecommunication components (e.g., wavelength-selective filters,multiplexers, demultiplexers, Mach-Zehnder interferometers, distributedBragg reflector lasers, pump/laser stabilizers, and supervisorychannels).

To supplement the present disclosure, this application incorporatesentirely by reference the following commonly assigned patents, patentapplication publications, and patent applications: U.S. Pat. No.4,838,643 for a Single Mode Bend Insensitive Fiber for Use in FiberOptic Guidance Applications (Hodges et al.); U.S. Pat. No. 7,623,747 fora Single Mode Optical Fiber (de Montmorillon et al.); U.S. Pat. No.7,587,111 for a Single-Mode Optical Fiber (de Montmorillon et al.); U.S.Pat. No. 7,356,234 for a Chromatic Dispersion Compensating Fiber (deMontmorillon et al.); U.S. Pat. No. 7,483,613 for a Chromatic DispersionCompensating Fiber (Bigot-Astruc et al.); U.S. Pat. No. 7,526,177 for aFluorine-Doped Optical Fiber (Matthijsse et al.); U.S. Pat. No.7,555,186 for an Optical Fiber (Flammer et al.); U.S. Pat. No. 8,055,111for a Dispersion-Shifted Optical Fiber (Sillard et al.); U.S. Pat. No.8,041,172 for a Transmission Optical Fiber Having Large Effective Area(Sillard et al.); International Patent Application Publication No. WO2009/062131 A1 for a Microbend-Resistant Optical Fiber, (Overton); U.S.Patent Application Publication No. US2009/0175583 A1 for aMicrobend-Resistant Optical Fiber, (Overton); U.S. Pat. No. 8,145,025for a Single-Mode Optical Fiber Having Reduced Bending Losses (deMontmorillon et al.); U.S. Pat. No. 7,889,960 for a Bend-InsensitiveSingle-Mode Optical Fiber, (de Montmorillon et al.); U.S. PatentApplication Publication No. US2010/0021170 A1 for a WavelengthMultiplexed Optical System with Multimode Optical Fibers, filed Jun. 23,2009, (Lumineau et al.); U.S. Pat. No. 7,995,888 for a Multimode OpticalFibers, (Gholami et al.); U.S. Patent Application Publication No.US2010/0119202 A1 for a Reduced-Diameter Optical Fiber, filed Nov. 6,2009, (Overton); U.S. Patent Application Publication No. US2010/0142969A1 for a Multimode Optical System, filed Nov. 6, 2009, (Gholami et al.);U.S. Patent Application Publication No. US2010/0118388 A1 for anAmplifying Optical Fiber and Method of Manufacturing, filed Nov. 12,2009, (Pastouret et al.); U.S. Patent Application Publication No.US2010/0135627 A1 for an Amplifying Optical Fiber and Production Method,filed Dec. 2, 2009, (Pastouret et al.); U.S. Patent ApplicationPublication No. US2010/0142033 for an Ionizing Radiation-ResistantOptical Fiber Amplifier, filed Dec. 8, 2009, (Regnier et al.); U.S.Patent Application Publication No. US2010/0150505 A1 for a BufferedOptical Fiber, filed Dec. 11, 2009, (Testu et al.); U.S. PatentApplication Publication No. US2010/0171945 for a Method of Classifying aGraded-Index Multimode Optical Fiber, filed Jan. 7, 2010, (Gholami etal.); U.S. Patent Application Publication No. US2010/0189397 A1 for aSingle-Mode Optical Fiber, filed Jan. 22, 2010, (Richard et al.); U.S.Patent Application Publication No. US2010/0189399 A1 for a Single-ModeOptical Fiber Having an Enlarged Effective Area, filed Jan. 27, 2010,(Sillard et al.); U.S. Patent Application Publication No. US2010/0189400A1 for a Single-Mode Optical Fiber, filed Jan. 27, 2010, (Sillard etal.); U.S. Patent Application Publication No. US2010/0214649 A1 for anOptical Fiber Amplifier Having Nanostructures, filed Feb. 19, 2010,(Burov et al.); U.S. Pat. No. 8,009,950 for a Multimode Fiber (Molin etal.); U.S. Patent Application Publication No. US2010/0310218 A1 for aLarge Bandwidth Multimode Optical Fiber Having a Reduced CladdingEffect, filed Jun. 4, 2010, (Molin et al.); U.S. Patent ApplicationPublication No. US2011/0058781 A1 for a Multimode Optical Fiber HavingImproved Bending Losses, filed Sep. 9, 2010, (Molin et al.); U.S. PatentApplication Publication No. US2011/0064367 A1 for a Multimode OpticalFiber, filed Sep. 17, 2010, (Molin et al.); U.S. Patent ApplicationPublication No. US2011/0069724 A1 for an Optical Fiber for Sum-FrequencyGeneration, filed Sep. 22, 2010, (Richard et al.); U.S. PatentApplication Publication No. US2011/0116160 A1 for a Rare-Earth-DopedOptical Fiber Having Small Numerical Aperture, filed Nov. 11, 2010,(Boivin et al.); U.S. Patent Application Publication No. US2011/0123161A1 for a High-Bandwidth, Multimode Optical Fiber with Reduced CladdingEffect, filed Nov. 24, 2010, (Molin et al.); U.S. Patent ApplicationPublication No. US2011/0123162 A1 for a High-Bandwidth,Dual-Trench-Assisted Multimode Optical Fiber, filed Nov. 24, 2010,(Molin et al.); U.S. Patent Application Publication No. US2011/0135262A1 for a Multimode Optical Fiber with Low Bending Losses and ReducedCladding Effect, filed Dec. 3, 2010, (Molin et al.); U.S. PatentApplication Publication No. US2011/0135263 A1 for a High-BandwidthMultimode Optical Fiber Having Reduced Bending Losses, filed Dec. 3,2010, (Molin et al.); U.S. Patent Application Publication No.US2011/0188826 A1 for a Non-Zero Dispersion Shifted Optical Fiber Havinga Large Effective Area, filed Jan. 31, 2011, (Sillard et al.); U.S.Patent Application Publication No. US2011/0188823 A1 for a Non-ZeroDispersion Shifted Optical Fiber Having a Short Cutoff Wavelength, filedJan. 31, 2011, (Sillard et al.); U.S. Patent Application Publication No.2011/0217012 A1 for a Broad-Bandwidth Multimode Optical Fiber HavingReduced Bending Losses, filed Mar. 1, 2011, (Bigot-Astruc et al.); U.S.Patent Application Publication No. 2011/0229101 A1 for a Single-ModeOptical Fiber, filed Mar. 15, 2011, (de Montmorillon et al.); U.S.Patent Application Publication No. 2012/0051703 A1 for a Single-ModeOptical Fiber, filed Jul. 1, 2011, (Bigot-Astruc et al.); U.S. PatentApplication Publication No. 2012/0040184 A1 for a Method of Fabricatingan Optical Fiber Preform, filed Aug. 10, 2011, (de Montmorillon et al.);U.S. Patent Application Publication No. 2012/0092651 A1 for a MultimodeOptical Fiber Insensitive to Bending Losses, filed Oct. 18, 2011, (Molinet al.); U.S. patent application Ser. No. 13/303,967 for aRadiation-Insensitive Optical Fiber Doped with Rare Earths, filed Nov.23, 2011, (Burov et al.); U.S. patent application Ser. No. 13/315,712for a Rare-Earth-Doped Optical Fiber, filed Dec. 9, 2011, (Boivin etal.); U.S. patent application Ser. No. 13/362,357 for a Broad-BandwidthOptical Fiber, filed Jan. 31, 2012, (Molin et al.); U.S. patentapplication Ser. No. 13/362,395 for a Multimode Optical Fiber, filedJan. 31, 2012, (Molin et al.); U.S. patent application Ser. No.13/410,976 for a Rare-Earth-Doped Amplifying Optical Fiber, filed Mar.2, 2012, (Burov et al.); U.S. patent application Ser. No. 13/428,520 fora Bend-Resistant Multimode Optical Fiber, filed Mar. 23, 2012, (Molin etal.); U.S. patent application Ser. No. 13/434,101 for a MultimodeOptical Fiber, filed Mar. 29, 2012, (Molin et al.); and U.S. patentapplication Ser. No. 13/456,562 for a High-Bandwidth,Radiation-Resistant Multimode Optical Fiber, filed Apr. 26, 2012,(Krabshuis et al.).

To supplement the present disclosure, this application furtherincorporates entirely by reference the following commonly assignedpatents, patent application publications, and patent applications: U.S.Pat. No. 5,574,816 for Polypropylene-Polyethylene Copolymer Buffer Tubesfor Optical Fiber Cables and Method for Making the Same; U.S. Pat. No.5,717,805 for Stress Concentrations in an Optical Fiber Ribbon toFacilitate Separation of Ribbon Matrix Material; U.S. Pat. No. 5,761,362for Polypropylene-Polyethylene Copolymer Buffer Tubes for Optical FiberCables and Method for Making the Same; U.S. Pat. No. 5,911,023 forPolyolefin Materials Suitable for Optical Fiber Cable Components; U.S.Pat. No. 5,982,968 for Stress Concentrations in an Optical Fiber Ribbonto Facilitate Separation of Ribbon Matrix Material; U.S. Pat. No.6,035,087 for an Optical Unit for Fiber Optic Cables; U.S. Pat. No.6,066,397 for Polypropylene Filler Rods for Optical Fiber CommunicationsCables; U.S. Pat. No. 6,175,677 for an Optical Fiber Multi-Ribbon andMethod for Making the Same; U.S. Pat. No. 6,085,009 for Water BlockingGels Compatible with Polyolefin Optical Fiber Cable Buffer Tubes andCables Made Therewith; U.S. Pat. No. 6,215,931 for FlexibleThermoplastic Polyolefin Elastomers for Buffering Transmission Elementsin a Telecommunications Cable; U.S. Pat. No. 6,134,363 for a Method forAccessing Optical Fibers in the Midspan Region of an Optical FiberCable; U.S. Pat. No. 6,381,390 for a Color-Coded Optical Fiber Ribbonand Die for Making the Same; U.S. Pat. No. 6,181,857 for a Method forAccessing Optical Fibers Contained in a Sheath; U.S. Pat. No. 6,314,224for a Thick-Walled Cable Jacket with Non-Circular Cavity Cross Section;U.S. Pat. No. 6,334,016 for an Optical Fiber Ribbon Matrix MaterialHaving Optimal Handling Characteristics; U.S. Pat. No. 6,321,012 for anOptical Fiber Having Water Swellable Material for Identifying Groupingof Fiber Groups; U.S. Pat. No. 6,321,014 for a Method for ManufacturingOptical Fiber Ribbon; U.S. Pat. No. 6,210,802 for Polypropylene FillerRods for Optical Fiber Communications Cables; U.S. Pat. No. 6,493,491for an Optical Drop Cable for Aerial Installation; U.S. Pat. No.7,346,244 for a Coated Central Strength Member for Fiber Optic Cableswith Reduced Shrinkage; U.S. Pat. No. 6,658,184 for a Protective Skinfor Optical Fibers; U.S. Pat. No. 6,603,908 for a Buffer Tube thatResults in Easy Access to and Low Attenuation of Fibers Disposed WithinBuffer Tube; U.S. Pat. No. 7,045,010 for an Applicator for High-SpeedGel Buffering of Flextube Optical Fiber Bundles; U.S. Pat. No. 6,749,446for an Optical Fiber Cable with Cushion Members Protecting Optical FiberRibbon Stack; U.S. Pat. No. 6,922,515 for a Method and Apparatus toReduce Variation of Excess Fiber Length in Buffer Tubes of Fiber OpticCables; U.S. Pat. No. 6,618,538 for a Method and Apparatus to ReduceVariation of Excess Fiber Length in Buffer Tubes of Fiber Optic Cables;U.S. Pat. No. 7,322,122 for a Method and Apparatus for Curing a FiberHaving at Least Two Fiber Coating Curing Stages; U.S. Pat. No. 6,912,347for an Optimized Fiber Optic Cable Suitable for Microduct BlownInstallation; U.S. Pat. No. 6,941,049 for a Fiber Optic Cable Having NoRigid Strength Members and a Reduced Coefficient of Thermal Expansion;U.S. Pat. No. 7,162,128 for Use of Buffer Tube Coupling Coil to PreventFiber Retraction; U.S. Pat. No. 7,515,795 for a Water-Swellable Tape,Adhesive-Backed for Coupling When Used Inside a Buffer Tube (Overton etal.); U.S. Patent Application Publication No. 2008/0292262 for aGrease-Free Buffer Optical Fiber Buffer Tube Construction Utilizing aWater-Swellable, Texturized Yarn (Overton et al.); European PatentApplication Publication No. 1,921,478 A1, for a TelecommunicationOptical Fiber Cable (Tatat et al.); U.S. Pat. No. 7,702,204 for a Methodfor Manufacturing an Optical Fiber Preform (Gonnet et al.); U.S. Pat.No. 7,570,852 for an Optical Fiber Cable Suited for Blown Installationor Pushing Installation in Microducts of Small Diameter (Nothofer etal.); U.S. Pat. No. 7,646,954 for an Optical Fiber TelecommunicationsCable (Tatat); U.S. Pat. No. 7,599,589 for a Gel-Free Buffer Tube withAdhesively Coupled Optical Element (Overton et al.); U.S. Pat. No.7,567,739 for a Fiber Optic Cable Having a Water-Swellable Element(Overton); U.S. Pat. No. 7,817,891 for a Method for Accessing OpticalFibers within a Telecommunication Cable (Lavenne et al.); U.S. Pat. No.7,639,915 for an Optical Fiber Cable Having a Deformable CouplingElement (Parris et al.); U.S. Pat. No. 7,646,952 for an Optical FiberCable Having Raised Coupling Supports (Parris); U.S. Pat. No. 7,724,998for a Coupling Composition for Optical Fiber Cables (Parris et al.);U.S. Patent Application Publication No. US2009/0214167 A1 for a BufferTube with Hollow Channels, (Lookadoo et al.); U.S. Patent ApplicationPublication No. US2009/0297107 A1 for an Optical Fiber TelecommunicationCable, filed May 15, 2009, (Tatat); U.S. Pat. No. 8,195,018 for a BufferTube with Adhesively Coupled Optical Fibers and/or Water-SwellableElement; U.S. Patent Application Publication No. US2010/0092135 A1 foran Optical Fiber Cable Assembly, filed Sep. 10, 2009, (Barker et al.);U.S. Pat. No. 7,974,507 A1 for a High-Fiber-Density Optical Fiber Cable(Louie et al.); U.S. Pat. No. 7,970,247 for a Buffer Tubes for Mid-SpanStorage (Barker); U.S. Pat. No. 8,081,853 for Single-Fiber Drop Cablesfor MDU Deployments (Overton); U.S. Pat. No. 8,041,167 for anOptical-Fiber Loose Tube Cables (Overton); U.S. Pat. No. 8,145,026 for aReduced-Size Flat Drop Cable (Overton et al.); U.S. Pat. No. 8,165,439for ADSS Cables with High-Performance Optical Fiber (Overton); U.S. Pat.No. 8,041,168 for Reduced-Diameter Ribbon Cables with High-PerformanceOptical Fiber (Overton); U.S. Pat. No. 8,031,997 for a Reduced-Diameter,Easy-Access Loose Tube Cable (Overton); U.S. Patent ApplicationPublication No. US2010/0154479 A1 for a Method and Device forManufacturing an Optical Preform, filed Dec. 19, 2009, (Milicevic etal.); U.S. Patent Application Publication No. US2010/0166375 for aPerforated Water-Blocking Element, filed Dec. 29, 2009, (Parris); U.S.Patent Application Publication No. US2010/0183821 A1 for a UVLEDApparatus for Curing Glass-Fiber Coatings, filed Dec. 30, 2009,(Hartsuiker et al.); U.S. Patent Application Publication No.US2010/0202741 A1 for a Central-Tube Cable with High-ConductivityConductors Encapsulated with High-Dielectric-Strength Insulation, filedFeb. 4, 2010, (Ryan et al.); U.S. Patent Application Publication No.US2010/0215328 A1 for a Cable Having Lubricated, Extractable Elements,filed Feb. 23, 2010, (Tatat et al.); U.S. Patent Application PublicationNo. US2011/0026889 A1 for a Tight-Buffered Optical Fiber Unit HavingImproved Accessibility, filed Jul. 26, 2010, (Risch et al.); U.S. PatentApplication Publication No. US2011/0064371 A1 for Methods and Devicesfor Cable Insertion into Latched Conduit, filed Sep. 14, 2010,(Leatherman et al.); U.S. Patent Application Publication No.2011/0069932 A1 for a High-Fiber-Density Optical-Fiber Cable, filed Oct.19, 2010, (Overton et al.); U.S. Patent Application Publication No.2011/0091171 A1 for an Optical-Fiber Cable Having High Fiber Count andHigh Fiber Density, filed Oct. 19, 2010, (Tatat et al.); U.S. PatentApplication Publication No. 2011/0176782 A1 for a Water-SolubleWater-Blocking Element, filed Jan. 19, 2011, (Parris); U.S. PatentApplication Publication No. 2011/0268400 A1 for a Data-Center Cable,filed Apr. 28, 2011, (Louie et al.); U.S. Patent Application PublicationNo. 2011/0268398 A1 for a Bundled Fiber Optic Cables, filed May 3, 2011,(Quinn et al.); U.S. Patent Application Publication No. 2011/0287195 A1for a Curing Apparatus Employing Angled UVLEDs, filed May 19, 2011,(Molin); U.S. Patent Application Publication No. 2012/0009358 for aCuring Apparatus Having UV Sources That Emit Differing Ranges of UVRadiation, filed Jun. 3, 2011, (Gharbi et al.); U.S. Patent ApplicationPublication No. 2012/0014652 A1 for a Adhesively Coupled Optical Fibersand Enclosing Tape, filed Jul. 13, 2011, (Parris); U.S. PatentApplication Publication No. 2012/0040105 A1 for a Method and ApparatusProviding Increased UVLED Intensity, filed Aug. 10, 2011, (Overton);U.S. Patent Application Publication No. 2012/0057833 A1 for anOptical-Fiber Module Having Improved Accessibility, filed Aug. 31, 2011,(Tatat); and U.S. patent application Ser. No. 13/401,026 for aOptical-Fiber Interconnect Cable, filed Feb. 21, 2012, (Risch et al.).

In the specification and/or figures, typical embodiments of theinvention have been disclosed. The present invention is not limited tosuch exemplary embodiments. The use of the term “and/or” includes anyand all combinations of one or more of the associated listed items. Thefigures are schematic representations and so are not necessarily drawnto scale. Unless otherwise noted, specific terms have been used in ageneric and descriptive sense and not for purposes of limitation.

1. A single-mode optical fiber, comprising: a central core surrounded byan outer optical cladding, said central core having a radius (R_(co)) ofbetween about 3.5 microns and 5.5 microns and a refractive-indexdifference with said outer optical cladding (Dn_(co)−Dn_(out)) ofbetween about −1×10⁻³ and 3×10⁻³; a first depressed cladding positionedbetween said central core and said outer optical cladding, said firstdepressed cladding having an outer radius (R_(cl1)) of between about 9microns and 15 microns and a refractive-index difference with said outeroptical cladding (Dn_(cl1)−Dn_(out)) of between about −5.5×10⁻³ and−2.5×10⁻³; and a second depressed cladding positioned between said firstdepressed cladding and said outer optical cladding, said seconddepressed cladding having an outer radius (R_(cl2)) of between about 38microns and 42 microns and a refractive-index difference with said firstdepressed cladding (Dn_(cl2)−Dn_(out)) of between about −0.5×10⁻³ and0.5×10⁻³; wherein said outer optical cladding has an outer radius ofbetween about 61.5 microns and 63.5 microns.
 2. The single-mode opticalfiber according to claim 1, comprising a third depressed claddingpositioned between said first and second depressed claddings, said thirddepressed cladding having a outer radius (R_(cl3)) of between about 15microns and 25 microns, a refractive-index difference with said firstdepressed cladding (Dn_(cl3)−Dn_(out)) of between about −0.5×10⁻³ and0.5×10⁻³, and a refractive-index difference with said second depressedcladding (Dn_(cl3)−Dn_(cl2)) of between about −0.5×10⁻³ and 0.5×10⁻³. 3.The single-mode optical fiber according to claim 1, wherein said centralcore has a refractive-index difference with said outer optical cladding(Dn_(co)−Dn_(out)) of between 0 and 3×10⁻³.
 4. The single-mode opticalfiber according to claim 1, wherein said central core consistsessentially of undoped silica.
 5. The single-mode optical fiberaccording to claim 1, wherein said outer optical cladding consistsessentially of undoped silica.
 6. The single-mode optical fiberaccording to claim 1, wherein the optical fiber has a cable cut-offwavelength (λ_(CC)) of 1260 nanometers or less.
 7. The single-modeoptical fiber according to claim 1, wherein, at a wavelength of 1550nanometers, the optical fiber has attenuation of less than about 0.18dB/km.
 8. The single-mode optical fiber according to claim 1, wherein,at a wavelength of 1383 nanometers, the optical fiber has attenuation ofless than about 0.35 dB/km.
 9. The single-mode optical fiber accordingto claim 1, wherein, at a wavelength of 1383 nanometers, the opticalfiber has attenuation of less than about 0.32 dB/km.
 10. The single-modeoptical fiber according to claim 1, wherein, at a wavelength of 1550nanometers, the optical fiber has macrobending losses of less than about1 dB/turn around a bend radius of 10 millimeters.
 11. The single-modeoptical fiber according to claim 1, wherein, at a wavelength of 1625nanometers, the optical fiber has macrobending losses of less than about0.05 dB/100 turns around a bend radius of 30 millimeters.
 12. A methodfor manufacturing an optical-fiber preform, comprising the steps of:depositing layers inside a deposition tube, the layers forming a centralcore and a first depressed cladding; completely removing the depositiontube; providing a second depressed cladding around the first depressedcladding; and providing an outer optical cladding around the seconddepressed cladding; wherein the ratio of the cross-sectional area of thecentral core to the cross-sectional area of the outer optical claddingis between about 0.0047 and 0.015; wherein the ratio of thecross-sectional area of the first depressed cladding to thecross-sectional area of the outer optical cladding is between about0.019 and 0.11; wherein the central core has a refractive-indexdifference with the outer optical cladding (Dn_(co)−Dn_(out)) of betweenabout −1×10⁻³ and 3×10⁻³; wherein the first depressed cladding has arefractive-index difference with the outer optical cladding(Dn_(cl1)−Dn_(out)) of between about −5.5×10⁻³ and −2.5×10⁻³; andwherein the second depressed cladding has a refractive-index differencewith the first depressed cladding (Dn_(cl2)−Dn_(cl1)) of between about−0.5×10⁻³ and 0.5×10⁻³.
 13. The method according to claim 12, comprisingthe step of drawing a single-mode optical fiber from the optical-fiberpreform; wherein the optical fiber has a central core with a radius(R_(co)) of between about 3.5 microns and 5.5 microns; wherein theoptical fiber has a first depressed cladding with an outer radius(R_(cl1)) of between about 9 microns and 15 microns; wherein the opticalfiber has a second depressed cladding with an outer radius (R_(cl2)) ofbetween about 38 microns and 42 microns; and wherein the optical fiberhas an outer optical cladding with an outer radius of between about 61.5microns and 63.5 microns.
 14. The method according to claim 12, whereinthe deposition tube consists essentially of undoped quartz.
 15. Themethod according to claim 12, wherein the second depressed cladding isprovided by sleeving with a doped tube and/or by outside deposition withdoped silica.
 16. A method for manufacturing an optical-fiber preform,comprising the steps of: depositing layers inside afluorine-doped-silica deposition tube, the layers forming a central coreand a first depressed cladding; providing a second depressed claddingaround the deposition tube, the deposition tube forming a thirddepressed cladding; and providing an outer optical cladding around thesecond depressed cladding; wherein the ratio of the cross-sectional areaof the central core to the cross-sectional area of the outer opticalcladding is between about 0.0047 and 0.015; wherein the ratio of thecross-sectional area of the first depressed cladding to thecross-sectional area of the outer optical cladding is between about0.019 and 0.11; wherein the ratio of the cross-sectional area of thesecond depressed cladding to the cross-sectional area of the outeroptical cladding is between about 0.31 and 0.77; wherein the ratio ofthe cross-sectional area of the third depressed cladding to thecross-sectional area of the outer optical cladding is less than about0.27; wherein the central core has a refractive-index difference withthe outer optical cladding (Dn_(co)−Dn_(out)) of between about −1×10⁻³and 3×10⁻³; wherein the first depressed cladding has a refractive-indexdifference with the outer optical cladding (Dn_(cl1)-Dn_(out)) ofbetween about −5.5×10⁻³ and −2.5×10⁻³; wherein the second depressedcladding has a refractive-index difference with the first depressedcladding (Dn_(cl2)−Dn_(cl1)) of between about −0.5×10⁻³ and 0.5×10⁻³;and wherein the third depressed cladding has (i) a refractive-indexdifference with the first depressed cladding (Dn_(cl3)−Dn_(out)) ofbetween about −0.5×10⁻³ and 0.5×10⁻³ and (ii) a refractive-indexdifference with the second depressed cladding (Dn_(cl3)−Dn_(cl2)) ofbetween about −0.5×10⁻³ and 0.5×10⁻³.
 17. The method according to claim16, comprising the step of drawing a single-mode optical fiber from theoptical-fiber preform; wherein the optical fiber has a central core witha radius (R_(co)) of between about 3.5 microns and 5.5 microns; whereinthe optical fiber has a first depressed cladding with an outer radius(R_(cl1)) of between about 9 microns and 15 microns; wherein the opticalfiber has a third depressed cladding with an outer radius (R_(cl3)) ofbetween about 15 microns and 25 microns; wherein the optical fiber has asecond depressed cladding with an outer radius (R_(cl2)) of betweenabout 38 microns and 42 microns; and wherein the optical fiber has anouter optical cladding with an outer radius of between about 61.5microns and 63.5 microns.
 18. The method according to claim 16, whereinthe second depressed cladding is provided by sleeving with a doped tubeand/or by outside deposition with doped silica.
 19. A method formanufacturing an optical-fiber preform, comprising the steps of:providing at least two successive cladding layers around a core rod, thecore rod forming a central core and the successive cladding layersforming a first depressed cladding and a second depressed cladding; andproviding an outer optical cladding around the second depressedcladding; wherein the ratio of the cross-sectional area of the centralcore to the cross-sectional area of the outer optical cladding isbetween about 0.0047 and 0.015; wherein the ratio of the cross-sectionalarea of the first depressed cladding to the cross-sectional area of theouter optical cladding is between about 0.019 and 0.11; wherein thecentral core has a refractive-index difference with the outer opticalcladding (Dn_(co)−Dn_(out)) of between about −1×10⁻³ and 3×10⁻³; whereinthe first depressed cladding has a refractive-index difference with theouter optical cladding (Dn_(cl1)-Dn_(out)) of between about −5.5×10⁻³and −2.5×10⁻³; and wherein the second depressed cladding has arefractive-index difference with the first depressed cladding(Dn_(cl2)−Dn_(cl1)) of between about −0.5×10⁻³ and 0.5×10⁻³.
 20. Themethod according to claim 19, comprising the step of drawing asingle-mode optical fiber from the optical-fiber preform; wherein theoptical fiber has a central core with a radius (R_(co)) of between about3.5 microns and 5.5 microns; wherein the optical fiber has a firstdepressed cladding with an outer radius (R_(cl1)) of between about 9microns and 15 microns; wherein the optical fiber has a second depressedcladding with an outer radius (R_(cl2)) of between about 38 microns and42 microns; and wherein the optical fiber has an outer optical claddingwith an outer radius of between about 61.5 microns and 63.5 microns. 21.The method according to claim 19, wherein the core rod is obtained byoutside deposition.
 22. The method according to claim 19, wherein thestep of providing at least two successive cladding layers around a corerod comprises providing at least three successive cladding layers aroundthe core rod, the successive cladding layers forming a first depressedcladding, a third depressed cladding, and a second depressed cladding.23. The method of claim 19, wherein the first depressed cladding and thesecond depressed cladding are provided by sleeving with a doped tubeand/or by outside deposition with doped silica.